1. Field of the Invention
The present invention generally relates to a power transfer mechanism, and more particularly, to a planetary gear type transmission incorporated in the power transfer mechanism.
2. Description of the Prior Art
The Japanese Patent Publication No. 62-7419 discloses a planetary gear type transmission as shown in FIG. 10. This transmission is comprised of an input shaft 1, a sun gear 2 rigidly secured to the input shaft 1, an output shaft 3, a carrier 4 splined to the output shaft 3 for rotation therewith and slidable in the direction axially of the output shaft 3, a plurality of planet gears 6 carried by the carrier 4 and a ring gear 5 carried by the planet gears 6. The planet gears 6 are in mesh with the sun gear 2 and the ring gear 5. The sun gear 2, the carrier 4, the planet gears 6 and the ring gear 5 constitute a planetary gear unit U.
The planetary gear unit U is shiftable between high and low speed conditions for direct drive and reduction ratio drive modes. In the direct drive mode, the planetary gear unit U is shifted rightwards, as shown in FIG. 10, so that the ring gear 5 may become free for prevention of excessive rotation of the planet gears 6. On the contrary, in the reduction ratio drive mode, the planetary gear unit U is shifted leftwards, as shown in FIG. 11, to lock the ring gear 5 to its casing 7 so that the planet gears 6 may be brought into sun-and-planet motion.
However, the change-over between the high and low speed conditions is done by shifting the entire planetary gear unit U, thus needing relatively large operation force and rendering the planetary gear unit to become complicated in construction.
The Japanese Utility Model Laid-Open Application No. 61-36748 discloses another planetary gear type transmission equipped with a planetary gear unit employing helical gears for its constituent rotary elements. Also, in this transmission, the engagement of the ring gear with respect to the casing is released in the high speed condition for the direct drive mode.
However, when the ring gear is made free in the direct drive mode, the sun gear and the ring gear rotate idly, thus undesirably producing noise between teeth of these gears and those of the planetary gears.